Combined effects of time and temperature on strength evolution using integral work-of-sintering concepts
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INTRODUCTION
POWDER metallurgy classically is applied to the fabrication of complex-shaped discrete components, formed to net shape. It is most useful as a manufacturing process for products requiring large production quantities, such as for components in appliances, business machines, industrial tools, and automobiles. As component shapes become more complicated, there is a barrier in final-dimension precision that reflects subtle process limitations. In our research on improved component precision, one of the underlying unknown factors is the relative importance of various influences on distortion. Green-density gradients are widely recognized as a factor,[1] and gravity also plays a role,[2,3] but there are several unknown influences. One case is heat transfer and the relative impact of thermal gradients on powder compacts during sintering. Compacted powder has a low thermal conductivity as compared to a sintered structure,[4] and finite-element analysis allows estimation of thermal stresses.[5] However, knowledge of the relative compact strength is required to determine the conditions under which these thermal stresses might contribute to component distortion. Our research on in situ strength evolution has focused on bronze powder compacts, in part because it is a simple metallurgical system free of phase transformations during sintering, yet widely employed in powder metallurgy. The earlier study isolated a transverse rupture test as being an appropriate means to measure strength during sintering without affecting the sintering trajectory.[6] In that study, prealloyed bronze powder was heated at 10 7C/min to various test temperatures and fractured. Additional tests were performed during cooling from these same peak temperaGREGORY A. SHOALES, formerly Research Assistant with the P/M Lab, Department of Engineering Science and Mechanics, The Pennsylvania State University, is Chief of the Materials Division, Engineering Mechanics Department, United States Air Force Academy, CO 80840-6240. RANDALL M. GERMAN, Brush Chair Professor in Materials, is with the P/M Lab, Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802-6809. Manuscript submitted February 3, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
tures, thereby illuminating the thermal softening character of the sintering cycle. Prior to significant sintering, annealing of the green compact leads to a minimum strength that is about one-half of the initial green strength prior to significant strengthening. Sinter strengthening is dependent on both time and temperature, but constant-heating-rate experiments did not allow separation of these factors. Over the past several years, many concepts have emerged for combining time-temperature kinetic models into a single integral term.[7] Recently, Johnson and Su[8] have advocated a similar concept for sintering cycles. Their approach has proven especially effective in rationalizing densification cycles for ceramics. In this study, we extend the study of st
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